PROJECT SUMMARY There have been substantial increases in the use of diverted buprenorphine (BUP) in the U.S. Research suggests that increases are driven by people self-treating opioid withdrawal. A growing trend of BUP diversion represents an important, although little understood, part of the larger prescription drug abuse problem in the U.S. that is intertwined with the soaring heroin epidemic. The overall purpose of this natural history study is to describe and analyze the nature and consequences of diverted BUP use among heroin and/or non-medical pharmaceutical opioid (NMPO) users who meet criteria for DSM-5 opioid use disorder. The study will be conducted in the Dayton, Ohio, Metropolitan Area, an emerging epicenter of the heroin epidemic. The study uses a mixed-methods approach. In Phase I (formative research) qualitative data will be collected to inform Phase II data collection. Phase II (sample recruitment and longitudinal follow-up) builds on a sequential explanatory mixed-methods design. We will recruit a community-based sample of 400 people with DSM-5 opioid use disorder who have used diverted BUP in the previous 90 days. The quantitative component will include structured baseline (N=400) and follow-up interviews conducted every six months for two years. The qualitative component will include 60 semi-structured in-depth qualitative interviews with individuals selected from the larger sample based on results of Latent Class Analysis (LCA) of baseline quantitative data. The Specific Aims are to: 1) Describe key dimensions of BUP diversion, motivations and patterns of BUP and other drug use, HIV/HBV/HCV risk behaviors, history of unintentional overdose, and drug abuse treatment use. 2) Identify trajectories of diverted BUP product use over 24 months and analyze their dynamic relationship to health-related outcomes, including opioid use/non-use, HIV/HBV/HCV risk behaviors, unintentional overdose, and drug abuse treatment engagement. Understanding the nature of diverted BUP use and its effects on opioid use and utilization of substance abuse treatment services has important clinical and public health implications. The study is highly significant because it will advance our understanding of crucial scientific research questions on the mechanisms of BUP diversion and relationships between trajectories of diverted BUP use, length of abstinence from other opioids, HIV risk behaviors, unintentional overdose, and drug abuse treatment seeking and engagement. It is highly innovative because it will be the first study to provide a longitudinal, mixed-methods approach to analyze BUP diversion and trajectories of use in the context of complex interactions between self-treatment behaviors and use of drug abuse treatment services. The mixed-method design is also novel because we will use LCA analysis results to select the qualitative sample. Our research questions will provide findings with critical public health impact that could be used to inform the design of interventions to address the NMPO/heroin epidemics and issues of BUP diversion.